1. Field of the Invention
This invention relates to optical control systems and devices, for example, touch sensitive displays and touch sensitive overlays for displays.
2. Description of the Related Art
In a traditional optical touch overlay, optical emitters and detectors are arranged along opposing edges of a rectangular touch sensitive area. An orthogonal grid is formed by the optical beams between the emitters and the corresponding detectors over the touch sensitive area. The beams are scanned in sequence to determine if the path loss indicates that there is an object (such as a finger or a stylus) interrupting or modulating any of the beams.
The touch sensitive area may be an optically transparent planar waveguide through which the beams pass by total internal reflection, or a surface over which the beams pass in close proximity to the surface and substantially parallel to the surface.
In the case of a waveguide, the material used can be a transparent sheet of plastic or glass. An object such as a finger or a stylus coming into contact with the transparent waveguide will have a higher refractive index than the air normally surrounding the waveguide. The increase in refractive index will disrupt total internal reflection of light energy at the interface between the waveguide and the touching object, causing increased light leakage from the waveguide over the area of contact. This leakage attenuates any beams passing through the position of the touch event. Correspondingly, the removal of the touching object will cause the attenuation of the passing beams to be reduced as will be detectable at the output of the associated detectors.
In the present specification the term “light” includes IR and UV radiation, and the term “optical” is to be interpreted accordingly.
When the touch sensitive area is a surface over which the beams pass, an object which is not optically transparent at the wavelength in use will attenuate or interrupt the beams passing through the object location.
This is illustrated in FIG. 1, where an opaque object 10 at position T attenuates a beam 12 between an emitter 14 on the left side of a touch sensitive area 16 and a detector 18 on the right side of the touch area 16. The opaque object 10 also interrupts a beam 20 between orthogonal axis emitters 22 and the corresponding detectors 24.
Scanning logic circuitry associated with the emitters and detectors can determine the intersection point of one or more interrupted beam paths and deduce the position of the interrupting object. The object must be large enough to detectably attenuate at least one beam in one axis and at least one beam in the orthogonal axis.
Such a panel traversed by optical paths can, in addition to the detection of touch events, detect the operation of mechanical control devices arranged along the optical paths. For example, a mechanical button control may be inserted along an optical path whereby the button control presents a minimal attenuation to optical energy when the button actuator is in its resting position, but introduces significant optical attenuation when the button actuator is pressed. This may be accomplished by introducing an opaque vane into the optical path when the button is pressed. Various mechanical designs for control devices may be used to modulate the optical energy passing along the optical paths such that the status of such control devices can be detected by processing the signals from the optical detectors.
The use of amplitude modulation of optical emitter outputs in optical touch and related optical control systems is well known. Typically, a specific frequency of modulation is chosen which can readily be separated from unwanted signals at the detector output by narrowband filtering. This modulating signal can be applied to an emitter by using it to modulate the current flowing through the emitter, which is usually be a current controlled device such as an LED.
However, optical emitter and detector response times (often tens of microseconds for detectors) and the need to be distant in frequency from the harmonics of mains-powered light sources limit the range of usable frequencies in low cost applications, so most devices using airborne optical transmission have optical emitters which are amplitude modulated by a fixed frequency in the 30 kHz to 80 kHz range.
Therefore, any device using optical detectors may receive external interference originating from other devices with emitters radiating light of a similar wavelength which is modulated by a similar range of frequencies.
Optical scanning devices normally activate emitters (modulated or unmodulated) in a sequential manner, so that the signal at one or more active detectors can be associated with a known source emitter. This prolongs the scanning process, particularly if modulation needs to be detected for each emitter signal to be determined reliably.